The Earlier button goes back all the way.

You can go directly to a page using the old-fashioned interface by appending ?tocpage=n to the URL to a particular forum. For example:

http://www.habitablezone.com/space/?tocpage=25

Try to use this sparingly. Large numbers force the server to compute TOCs (figuring out how many threads fit a page) starting at the front and working all the way back to the number of pages you specify. It's not a big deal when you're browsing back one TOC at a time, becaues the software just computes the next page incrementally and caches the TOCs it previously generated. But in one big gulp you could tie up the server for a long time.

To put it in perspective, the example above going back 25 pages took me back to just May 3rd, and took about 15 seconds. 500 pages took nearly three minutes to compute, and went back only as far as the end of April. This is obviously not a very good solution for what you want, but the best I can come up with on short notice. Sorry about that.